Proton Exchange Membrane (PEM) fuel cells offer a means to generate power with zero carbon emissions and high efficiency. Extensive research has shown that metal hydrides (MHx) can provide a safe, reliable, and pure supply of hydrogen for a PEM fuel cell versus compressed or cryogenic hydrogen. In these systems, the metal hydride component is considered a “one-time-use” material and is not rehydrogenated due to unfavorable rehydrogenation kinetics. This type of system is not currently practical for large scale automotive applications; however, it is ideal for portable power applications such as military use and use by emergency responders.
Sodium borohydride (NaBH4) has been studied extensively as a hydrogen source for portable power systems due to its high theoretical hydrogen content.NaBH+2H2O→NaBO2+4H2; 10.8 wt %   (1)
However, an NaBH4 based system has a disadvantage of not releasing all the hydrogen due to the formation of hydroxide layer around the NaBH4 particles.
The release of hydrogen from NaBH4 proceeds through an aqueous process that utilizes heat and a transition metal catalyst (Ti, Co, or Ru based) for the hydrolysis reaction to occur. The use of water in the aqueous process reduces the available hydrogen for applications to only five percent. Other work has shown that the methanolysis of NaBH4 is another possible route for hydrogen release. The use of methanol also allows for the system to operate at sub-zero temperatures if necessary, but also requires the use of a transition metal catalyst. Typical experimental conditions for the NaBH4 systems include a large excess of water due to the poor solubility of the NaBO2 hydrolysis product in water (28 g/100 g water) and limits the starting concentration of NaBH4 to below 16 g/100 g of H2O. Also, the calculation of the H2 capacity is often based on the identity of the hydrolysis product (i.e. NaBO2.4H2O, 7.3 wt % H2; NaBO2.6H2O, 5.5 wt % H2) recovered from solution after the hydrolysis reaction and does not account for the large excess of water (or methanol) needed for the reaction to occur. Aluminum hydride (AlH3), while not a new material has only in the last few years been notably considered as a hydrogen storage material for fuel cell applications due to its high volumetric and gravimetric hydrogen capacities (148 g/L and 10.1 wt % respectively) as well as its favorable desorption kinetics. There are many polymorphs of AlH3 (α, {acute over (α)}, β, δ, ε, Υ, ζ), however, the most studied has been the α polymorph due to its stability and will be the focus of this study. A major drawback to the use of this material is that rehydrogenation of aluminum requires in excess of 105 bar H2 and is currently impractical for automotive applications.
Accordingly, there remains room for improvement and variation in the art.